Periodic builder stops being assigned to slave

Description

Maybe related to #85: I have a single master and single slave, and 4 once-per-minute periodic builds. The master and slave have been running for a few days, and at some point ONE of the periodic builds stopped being assigned (I think that's the problem), while the other 3 builds are continuing once per minute as normal. Pending builds for the builder (called "send_confirmation_emails_and_smses") have stacked up so that there are thousands pending.